Extends resolve_xcom_backend function level documentation (#24965)

* # This is a combination of 2 commits.
# This is the 1st commit message:

Extends resolve_xcom_backend documentation
# This is the commit message #2:

properly stylize XCom

* Extends resolve_xcom_backend documentation

properly stylize XCom

Final docstring update

Extends resolve_xcom_backend documentation

properly stylize XCom

Final docstring update

* Stylistic changes

(cherry picked from commit 178af9d)

airflow/models/xcom.py

@@ -658,7 +658,11 @@ def _get_function_params(function) -> List[str]:
 
 
 def resolve_xcom_backend() -> Type[BaseXCom]:
-    """Resolves custom XCom class"""
+    """Resolves custom XCom class
+
+    Confirms that custom XCom class extends the BaseXCom.
+    Compares the function signature of the custom XCom serialize_value to the base XCom serialize_value.
+    """
     clazz = conf.getimport("core", "xcom_backend", fallback=f"airflow.models.xcom.{BaseXCom.__name__}")
     if not clazz:
         return BaseXCom